Known wind turbines have a rotor generally comprising a hub and three turbine blades mounted on the hub. The load on the rotor may be controlled by pitching the blades, i.e. by rotating each blade around its longitudinal axis (from root to tip) and cause the wind to engage a larger or smaller surface of the blade.
For the purpose of pitching the blades, each blade is mounted on the hub by means of a pitch system, which in known turbines comprises a pitch bearing arranged between the hub and the blade, and a pitch drive which may include a motor, a drive pinion, and an annular gear meshing with the pinion and causing the blade to rotate and change its pitch angle, upon rotation of the drive pinion.
A known pitch bearing has one of the bearing races, e.g. the outer race, connected to the hub, and the other bearing race, e.g. the inner race, connected to a blade root portion (or sometimes to an extender); the annular gear of the pitch drive may then be fixed to the inner race of the pitch bearing, such that its rotation causes rotation of the inner race of the bearing and of the blade attached thereto.
For attachment of the pitch bearing to the blade root portion and to the hub, it is known to provide a plurality of studs, screws, or similar elements, each extending through a bore in one of the bearing races and screwed or fixed in any other way to the hub or to the blade root portion. It is known that such studs need to withstand fatigue loads and that long studs are better suited to withstand fatigue than short studs, and it is therefore desirable to provide a pitch bearing construction that allows a sufficient stud length.
On the other hand, pitch bearings must stand several forces and moments, and in particular the bending moment caused by the force of the wind on the blades, that tends to strain especially the region of the bearing remote from the hub. One known construction employs a pitch bearing having two or three rows of rolling elements, usually balls, in order to transfer the torque from the blades to the hub and at the same time withstand bending loads.
However, the axial length of such pitch bearings is relatively large, and at the same time they need to have a suitable stiffness to withstand the loads; their construction is therefore relatively complex, and their cost rather high.
It would be desirable to provide a pitch system for a wind turbine rotor that is cost effective and of relatively simple construction.